In vitro iron enrichment experiments in the NE subarctic Pacific

Abstract

There has been considerable debate about both the mechanisms controlling primary production, and the interpretation of data from Fe-enrichment experiments conducted in high nitrate-lotv chlorophyll (HNLC) regions. This paper presents results of 3 in vitro Fe-enrichment experiments performed in May 1993 and May 1994 at Ocean Station Papa (OSP) in the NE subarctic Pacific. Expt 1 (May 1993) considered jointly the influence of both Fe supply and microzooplankton herbivory on algal stocks, while the second (May 1993) investigated the influence of Fe supply on the partitioning of C, N and Fe between algal size classes. The third study (May 1994) monitored changes in phytoplankton stocks in Fe-enriched carboys containing mesozooplankton Assuming simllar environmental conditions in May 1993 and 1994, then the comblned findings indicate that under ambient conditions autotrophic cells < S pm compose >70'% of algal biomass, primarily utilized ammonium, showed no Femediated enhancement of N-normalized nitrate or ammonium uptake, and were thus unlikely to be Felimited. Although the coupling between grazer mortality and algal growth of these cells ( g / p ) was ca 0.71 k 0.25, they grew at close to maximal rates but exhibited negligible net growth rates, suggesting strong grazer control. Dissolved iron (DFe) supply promoted a rapid increase in the abundance of large, initially rare, diatoms (mainly Njtzschla spp.) over 6 d, elevated diatom growth rates to close to their theoretical maximum and increased rates of incorporation of N-normalized nitrate and Fe by these cells over time. Unlike the small cells, stocks of large autotrophs also increased in the controls; DFe measurements indicate that this was probably due to inadvertent Fe contamination. The development of a phytoplankton population, of a cell size probably too large to be grazed at a significant rate by microzooplankton, resulted in a decoupling of herbivory and algal growth. In addition, mesozooplankton herbivory appeared unable to prevent the accumulation of these large cells; in May 1994, Fe supply prompted an 8-fold increase in algal stocks over 6 d in carboys where grazers equivalent to the maximum annual in situ pelagic abundance were present. The findings agree with the theories of others, that Fe supply rather than grazing provides the ultimate control over the phytoplankton community in the NE subarctic Pacific.

Seasonal and interannual variability of phytoplankton, nutrients, TCO2, pCO2, and O2 in the eastern subarctic Pacific (ocean weather station Papa)

“…In addition to these modifications, two new ecosystem components, iron (Fe) and dissolved organic carbon (DOC), were added to the model. Previous studies [Martin et al, 1989;Boyd et al, 1996;Maldonado et al, 1999] showed that phytoplankton growth in the subarctic Pacific is generally iron limited. We added the iron component to obtain more accurate simulations of nitrate, phytoplankton, and community production.…”

Atmospheric iron supply and enhanced vertical carbon flux in the NE subarctic Pacific: Is there a connection?

“…In vitro iron enrichments at OSP result in elevated phytoplankton biomass and production, and a floristic shift towards large diatoms [Martin et al, 1987;Boyd et al, 1996]. The magnitude of phytoplankton production [Bishop, 1989], and cell size [Michaels and Silver, 1988] In contrast to Asia, the probability of transport from Alaska is less variable through the year, with higher values in summer and fall months than for Asia ( Figure 5).…”

Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

“…Ship-board Fe addition incubation experiments have been widely used to assess the importance of iron as a factor limiting the growth of phytoplankton in HNLC waters (Boyd et al, 1996;Boyd et al, 1998;Coale et al, 2003;Cochlan et al, 2002;Fitzwater et al, 1996;Martin and Fitzwater, 1988;Timmermans et al, 1998;Zettler et al, 1996). Noiri et al (2005) conducted ship-board Fe addition experiments during the SEEDS experiments in 2001.…”